According to the PMBOK Guide, the first step in conflict resolution is to understand the root cause of the conflict. In this case, the project manager should first have a direct conversation with the functional manager to understand their perspective and the reasons behind their reluctance to release the staff. This approach aligns with the PMI's emphasis on communication and stakeholder engagement as key aspects of project management. References: PMBOK Guide, 6th Edition, Chapter 9: Project Resource Management, Section 9.5:
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